This application claims the priority of Korean Patent Application No. 2002-7011, filed Feb. 7, 2002, which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a semiconductor optical device and a method for manufacturing the same, and more particularly, to a semiconductor optical device having a differential grating structure and a method for manufacturing the same.
2. Description of the Related Art
Functional optical devices, which select a specific wavelength in a wavelength division multiplexing (WDM) network, including a distributed feedback (DFB) laser diode having a narrow optical spectrum, which is required to increase a transmission distance in optical communications, perform a wavelength filtering operation by using gratings and various kinds of grating structures have been developed and published. In a semiconductor optical device, a periodical variation of the refractive index of lightwave, which progresses toward the optical device, is affected by a wavelength filtering operation. Thus, a specific wavelength corresponding to a Bragg wavelength is reflected and fed back to a gain region in the semiconductor optical device in order to operate a specific wavelength. Such a semiconductor optical device is manufactured by performing a plurality of processes, for example, growth and regrowth of a semiconductor material, formation of a grating, dry and wet etchings, and formation of electrodes. Especially, a holography method for attaining a uniform grating on a large area is commonly used in the formation of a grating. In order to constitute a grating in a special shape, an electron-beam lithography method may be used. When using the electron-beam lithography method, optical filtering characteristics of the optical device are attained, but an electron-beam direct writing method of the electron-beam lithography method increases a process time, which causes a limit in the increase of the yield of the devices. Accordingly, most of optical devices including a grating are manufactured by the holography method.
Demands for optical devices having special functions, such as a self-pulsating DFB laser diode for generating and processing high-speed optical signals, have increased recently. In manufacturing such optical devices, the conventional holography method has limits in constituting a grating. For example, U.S. Pat. No. 6,215,805B1 discloses a Q-switched semiconductor laser having two reflectors with different periods. In order to manufacture the Q-switched semiconductor laser, gratings with different periods have to be formed on an active layer and a reflective layer, respectively. In this case, the gratings cannot be formed by the conventional holography method. Actually, the above US patent constituted the reflectors with different reflective characteristics by forming gratings with different periods on the active and reflective layers by the electron-beam lithography method. Accordingly, in order to manufacture special gratings, for example, the gratings with different periods and refractive indexes, the electron-beam lithography method has been used. In this case, the direct writing method limits the yield of the optical devices, which causes a limit in ensuring commercial availability of the optical devices.
As a result, a method for manufacturing various kinds of optical devices by forming gratings by a holography method is required to improve the yield of optical devices.